1. Field of the Invention
The present invention is directed to melt-to-mold processes for preparation of molded articles of crystallizable polyester, to compositions suitable for use therein, and to molded articles produced thereby.
2. Background Art
Ovenable food trays of crystallizable polyester have become significant items of commerce. While food trays such as those employed in prepackaged meals can, in principle, be prepared from numerous polymers, it is desired that such trays be suitable both for use in conventional ovens and in microwave ovens (“dual ovenable”). In addition, at the temperature of the hot food, such trays must exhibit dimensional stability for acceptable handling. For these reasons, crystallizable polyesters are the predominant construction material due to their high melting point and excellent dimensional stability.
Two processes are in use for the thermoforming of food trays from crystalline polyester, and the physiochemical properties of the polyester compositions used in these processes are significantly different. In the first process, sometimes termed the “roll-fed” or “in line” process, as disclosed in U.S. Pat. No. 3,496,143, the thermoforming process both forms the shape of the tray and crystallizes the polyester, which is supplied as a vitrified (amorphous) film. Polyester obtained from the melt is amorphous, and development of significant crystallinity is necessary to obtain the desired physical properties. In this first process, amorphous polyester sheet (film) is heated, and then supplied to a heated mold, for example a mold formed between two heated platens. Crystallization is then accomplished by holding the polyester at a temperature between its glass transition temperature, Tg, and its crystalline melt temperature, Tm. Crystallization of the sheet in its net shape produces the desired high temperature stability of the thermoformed article, and allows its removal from the mold without damage. Thus, in this first process, the polyester is heated from below its glass transition temperature to a temperature range in which crystallization can occur.
The foregoing process requires preparation and storage of an amorphous polyester film. Unmodified, crystallizable polyesters such as polyethylene terephthalate (PET) crystallize slowly when cooled from the melt or heated from below the glass transition temperature. To obtain acceptable manufacturing economics, it is necessary that the rate of thermal crystallization in the mold be rapid. However, at the same time, the crystallization rate upon cooling from the melt must be such that an amorphous film can be prepared.
A well known method of increasing the crystallization rate of polyesters in general is incorporation of a crystallization nucleator into the polyester, typically inorganic or organic solids which are finely dispersed therein. An example of an inorganic nucleator is talc, while an example of an organic nucleator is polyethylene. However, these nucleators are typically used in injection molding processes, where crystallization occurs during cooling from the melt, and rapid crystallization is the desired goal. Such nucleators may also induce rapid crystallization as the polymer is heated from below the Tg as well, and polyethylene, for example, is the dominant nucleator used in roll-feed operations. However, the operability of any given injection molding nucleator in thermoforming processes is unpredictable.
In the second of the thermoforming processes, to which the subject invention is directed, the polyester sheet is extruded directly before thermoforming, and is thermoformed prior to complete vitrification. This process is termed the melt-to-mold process. In contrast to the roll-fed process where the polyester sheet is heated from below its Tg, in the melt-to-mold process, the polyester is at or above its Tg. Thus, the crystallization process is completely different, and it has been found, in general, that crystallization nucleators eminently suitable for the roll-fed process are ill-suited for the melt-to-mold process. The differences in crystallization due to the thermal history of the polyester is discussed by D. W. van Krevelen, CHIMIA, 32 (1978), p. 279, where large differences in nucleation density are observed with differences in thermal history, i.e. depending upon whether the polymer is heated from below the glass transition temperature or cooled from the melt to the crystallization temperature.
In the melt-to-mold process, typical nucleators such as those employed in injection molding are not effective, as they often induce crystallinity rapidly and at an uncontrolled rate. While such nucleators may be eminently successful for injection molded parts, in the melt-to-mold process, the film should not appreciably crystallize prior to thermoforming. On the other hand, for economical processing, it is necessary that the thermoformed article rapidly but controllably crystallize in the mold. Thus, the requirements of successful nucleators in the melt-to-mold process are very critical.
The selection of crystallization nucleators in thermoforming of crystallizable polyesters is further complicated by the additives generally employed. Such additions typically include fillers, pigments, and most importantly, impact modifiers. In the roll-fed thermoforming process, for example, as disclosed in U.S. application Ser. No. 10/135,628, use of polyolefin nucleating agents together with polyolefin impact modifiers as taught by U.S. Pat. No. 3,960,807, produces negative effects in the crystallization rate of the polyester. Thus, the chosen crystallization nucleator must operate successfully in polyesters containing other ingredients which may affects its performance.
Nucleators which facilitate crystallization and have been used in polyester molding and roll-fed thermoforming processes include poly(tetramethylene terephthalate) polyesters, as disclosed in copending U.S. application Ser. No. 10/135,628; metal salts of polyesters as disclosed by U.S. Pat. No. 5,405,921; combinations of inorganic compounds with polyester compositions having specific end group chemistry as disclosed in U.S. Pat. No. 5,567,758; sodium compounds and wax, as disclosed in U.S. Pat. No. 5,102,943; poly(butylene terephthalate), copolyetheresters, or nylon 6,6, as disclosed in Research Disclosure 30655 (October 1989); polyester elastomers in polyethylenenaphthalate polyesters, as disclosed in U.S. Pat. No. 4,996,269; poly(oxytetramethylene) diol, as disclosed in U.S. Pat. No. 3,663,653; ethylene-based ionomers in block copolyesters as disclosed in U.S. Pat. No. 4,322,335; polyoxyalkylene diols as disclosed in U.S. Pat. No. 4,548,978; alkali metal salts of dimer or trimer acids, as disclosed in U.S. Pat. No. 4,357,268; sodium salts of fatty acids in conjunction with alkyl esters of a C2-8 carboxylic acid as disclosed in U.S. Pat. No. 4,327,007; partially neutralized salts of a polymer containing neutralizable groups, as disclosed in U.S. Pat. No. 4,322,335; neutralized or partially neutralized salts of montan wax or montan wax esters as disclosed in U.S. Pat. No. 3,619,266; epoxidized octyloleate together with sodium stearate, as disclosed in U.S. Pat. No. 4,551,485; and amino-terminated polyoxyalkylene polyethers as disclosed in U.S. Pat. No. 5,389,710. However, a nucleating agent which is useful for melt-to-mold thermoforming has neither been disclosed, nor taught or suggested by these references.
Large quantities of polyester, particularly PET, is used in the manufacture of beverage containers. The properties of the polyester employed are considerably different from those of polyester used for thermoforming. The polyester employed for beverage containers is generally required to have high brightness and clarity, and the “p reforms” or “parisons” used to blow mold the beverage containers are injection molded. Because of the brightness and clarity requirements, particulate nucleating agents and impact modifiers are generally absent, since their presence will cause a haze or cloudiness of the product. In such polyesters, a more exacting requirement is a low acetaldehyde content, both as produced and in the parisons molded therefrom. The search for effective polycondensation catalysts which allow for reasonable rates of polycondensation while limiting acetaldehyde generation, identification of catalyst deactivators which minimize acetaldehyde generation during molding, and acetaldehyde “scavengers” which scavenge acetaldehyde or prevents its migration into food products is a subject of considerable on-going development.
For example, phosporic acid has been used as a catalyst deactivator in antimony-catalyzed polycondensation, but must be added carefully to avoid production of precipitants which lower clarity. Titanium catalysts are much more effective polycondensation catalysts, but generally produce a product with higher yellowness, and thus, to date, have been seldom used. In such systems, organophosphorus compounds such as trimethylphosphate, triethylphosphate, and triphenylphosphate have been touted as deativators, added late in the melt-phase polycondensation. In U.S. Pat. No. 4,837,115, addition of high molecular weight polyamides is disclosed as lowering acetaldehyde, while in U.S. Pat. No. 5,258,233, addition of 0.05 to 2.0 weight percent of an aromatic polyamide with a molecular weight below 15,000 g/mol or an aliphatic polyamide with a molecular weight below 7000 g/mol is disclosed. The latter patent also discloses thermoformable polyester sheet (roll-fed process) which employs polyethylene as a crystallization nucleator.
It would be desirable to provide a crystallization nucleator which is effective in melt-to-mold thermoforming of crystallizable polyester. It would be further desirable to provide a crystallization nucleator which allows for tailoring of the rate of crystallization, and which is suitable for use with additives typically employed in polyesters used to prepare thermoformed products by the melt-to-mold process.